My long-term research goal is to understand the cellular mechanisms by which neural networks control animal behaviors. It is also becoming clear in many systems that the final motor response of an animal to a given sensory stimulus is significantly influenced by not only excitatory interactions in the nervous system, but also by inhibitory interactions. In several invertebrates it has been suggested that the initiation of a specific rhythmic behavior may not only depend on neurons that excite the appropriate neural network, but also requires disinhibition of inhibitory inputs to the neural network. Even higher order behaviors, such as behavioral choice, involve the inhibitory interactions between central neural networks. Thus to fully understand how an animal determines the appropriate motor response to a given sensory stimulus it is essential to investigate how the nervous system integrates both excitatory and inhibitory inputs at the neural network level. I propose to identify swim inhibitory neurons (SINs) in the head ganglion. To facilitate searching for SINs, I will use an ionic manipulations technique which is based on the fact that an increase in the extracellular K4+ concentration depolarizes neurons and an increase in the extracellular Cl- concentration hyperpolarizes neurons. Each SIN identified will be physiologically characterized by their input and output connections to the segmental swim generating network. In addition, I plan to examine the role SINs play in determining whether stimulation of cell Trl triggers swimming behavior. Variations in the activity levels of SINs will be compared in swimming and non-swimming trials to establish whether SIN activity is predictive of Trl induced swimming behavior. This research thus will directly examine how the leech nervous system integrates excitatory and inhibitory inputs to regulate swimming behavior. Similar integrative properties of the nervous system are likely used by other animals to control their behavioral responses to environment signals.